It is often necessary to characterize the performance of an electrical device with respect to certain operational parameters of the device. This characterization may be referred to as “parametric testing”. One method by which to measure the performance of a device is on a pass/fail basis. A complete parametric test would provide a pass/fail result for every possible combination of all values of the operational parameters in question. This type of test provides users of the device information as to how the device will perform over a broad range of test conditions.
A shmoo plot is a graphical representation of the ability of an electrical device to operate properly in response to various combinations of values of two variable operating parameters, or “test conditions”. For example, an integrated circuit (“IC”) device might be repeatedly tested using different combinations of supply voltage and CPU clock signal frequency to determine the various test conditions in which the IC operates properly.
FIG. 1 illustrates a conventional shmoo plot 100. Each of the X- and Y-axes, designated 102(X) and 102(Y), respectively, represents the value of a test operating parameter. Continuing with the IC example set forth above, the X-axis 102(X) represents the value of supply voltage (Vcc) in units of volts (“V”) and the Y-axis 102(Y) represents CPU clock signal frequency in units of megahertz (“Mhz”). To generate sufficient data to produce a useful shmoo plot, a device must be tested at an adequate number of combinations of X and Y operating parameter values within a range of interest bounded by Xmin, Xmax, Ymin, and Ymax, with some predetermined resolution in the X and Y interval size. Again, returning to the IC example, as illustrated in FIG. 1, Xmin and Xmax are 1.0 V and 2.1 V, respectively, while Ymin and Ymax are 470 Mhz and 1100 Mhz, respectively. The X and Y interval sizes are 0.1 V and 30 Mhz, respectively.
A “pass” symbol, represented in FIG. 1 by the symbol “---”, is plotted when the device passes a test performed under the combination of operating parameters identified by the corresponding Cartesian (X, Y) coordinate pair. Similarly, a “fail” symbol, represented in FIG. 1 by the symbol “xxx”, is plotted when the device fails a test performed under the combination of operating parameters identified by the corresponding Cartesian (X, Y) coordinate pair. In the IC example used herein, the IC is deemed to have “passed” if it functions correctly under a given set of test conditions.
As evidenced by the shmoo plot 100, shmoo plots provide a clear depiction of the operational limits of a device under various test conditions. A passing region 104 comprises the collection of passing points. A failing region 106 comprises the collection of failing points. An observer can easily note where the performance of the device transitions from the passing region 104 to the failing region 106 as the operating parameters are varied. It will be recognized, however, that a significant amount of time is required to perform the tests that form each of the individual array elements of a shmoo plot. In a simple 16×16 shmoo plot, assuming each test requires 100 μs to perform, the entire shmoo plot would require 16×16×100 μs, or 25.6 ms, to complete. Realistically, each operating parameter will be swept over a range of 100 values and each individual test could take on the order of a second to complete. Accordingly, a shmoo plot corresponding to such a parametric test would take nearly three hours to complete.